The invention relates to moulded bodies of titanium dioxide having a mean grain size d50 of 0.01 to 50 mm, a process for production thereof and use thereof.
Such titanium moulded bodies, which are generally used for fixed bed or fluidised bed systems, must have a sufficiently high strength since otherwise the catalyst will disintegrate when packed in the reactor and/or becomes comminuted or subjected to severe abrasion due to the gas pressure, thermal stress and dynamic loading during the reaction.
U.S. Pat. No. 4,113,660 relates to a process for the production of moulded bodies having improved mechanical strength and consisting largely of titanium dioxide, for use as catalysts or as catalyst supports. These moulded bodies are produced by subjecting titanium dioxide or a precursor substance of titanium dioxide to a calcining treatment at temperatures of 200xc2x0 to 800xc2x0 C. in the presence of metatitanic acid sol or gel. The subject of U.S. Pat. No. 4,061,596 is a process for the production of titanium dioxide moulded bodies in which titanium dioxide is calcined at temperatures of 4000 to 800xc2x0 C. and a mineral acid or carboxylic acid or aluminium oxide or an intermediate of aluminium oxide selected from aluminium hydroxide, aluminium oxide sol and aluminium salts is added to the calcined titanium dioxide. The moulded bodies produced from this mixture are subjected to a calcining treatment at 300xc2x0 to 800xc2x0 C. The moulded bodies treated in this way are said to have an improved mechanical strength. In DE-C-41 41 936 a process is described for the production of pellets consisting of titanium dioxide, in which metatitanic acid or orthotitanic acid without additives is treated at a temperature of 120xc2x0 to 250xc2x0 C. and under a pressure of 2 to 40 bar for a duration of 0.1 to 30 hours in the presence of water and is then dried. This substance is then formed into pellets under the addition of water in a pelletising device and the pellets are treated at a temperature of 100xc2x0 to 1000xc2x0 C. A disc pelletiser or a rotary drum, for example, serves as pelletising device, the hydrothermally treated metatitanic acid or orthotitanic acid being added to the respective pelletising device and being granulated while spraying with water. The produced pellets have a relatively high compression strength of, on average, 3 to 10 N, so that they can be used as bed material in fixed bed as well as in fluidised bed systems.
The object of the present invention is to provide moulded bodies consisting of titanium dioxide with a mean grain size d50 of 0.01 to 50 mm that have an improved compression strength, in which the proportion of foreign ions is extremely low, whose abrasion resistance is large, and that can be produced with normal expenditure and effort without using binders.
This object is achieved by moulded bodies of titanium dioxide that are composed in each case of primary crystallites of titanium dioxide in the anatase modification with a crystallite size according to Scherrer of up to 40 nm, preferably 10 to 30 nm, whose specific surface is 20 to 150 m2/g, whose pore volume is 0.1 to 0.45 cm3/g, and whose pore diameter is 100 to 300 xc3x85.
The best compression strength is achieved if primary crystallites either with a grain size of 4 to 10 mm or with a grain size of 10 to 15 mm are used for the moulded bodies.
The production of the moulded bodies consisting of titanium dioxide is carried out according to the invention by drying the moulded bodies produced from a paste-like mixture of titanium oxide powder and titanium oxide sol consisting of primary crystallites and/or 1 to 15% nitric acid and then annealing the moulded bodies at a temperature of 400xc2x0 to 1000xc2x0 C. for up to 3 hours, preferably 0.5 to 2.5 hours. The crystallite size after the annealing is 10 to 40 xcexcm.
For the purposes of the preferred modification of the invention, the paste-like mixture consists of 1 to 50 wt. % of titanium dioxide powder and 50 to 99 wt. % of titanium dioxide sol and/or 1 to 15% nitric acid.
Having regard to the comparatively high abrasion resistance of the moulded bodies it has proved extremely advantageous if the titanium dioxide powder is, before the preparation of the paste, pre-processed at a temperature of 120xc2x0 to 250xc2x0 C. and under a pressure of 2 to 40 bar for a period of 0.1 to 30 hours in the presence of water and is then dried.
The moulded bodies according to the invention consisting of titanium dioxide can also be produced by forming the titanium dioxide powder consisting of primary crystallites into pellets with the addition of water, annealing the pellets at temperatures of 300xc2x0 to 500xc2x0 C., then impregnating them with titanium dioxide sol and/or 1 to 15% nitric acid, next drying the pellets and finally annealing the pellets at a temperature of 400xc2x0 to 1000xc2x0 C. for up to 3 hours, preferably 1 to 2 hours.
The advantages achieved with the invention are in particular that the resistance to crushing of the cylindrical and prism-shaped moulded bodies is in each case at least 7 N/mm, that of the spherical moulded bodies is in each case at least 15 N, and the abrasion resistance is  less than 1%.